An X-ray CT (Computed Tomography) device is a device for calculating an X-ray absorption rate of each point from X-ray projection data obtained by imaging a subject from many directions to obtain an distribution image of X-ray absorption rate (hereinafter, referred to as a CT image) consisting of a plurality of pixels as a tomographic image of the subject. The CT image obtained by the present device can accurately and rapidly diagnose a patient's illness and is useful clinically in medical settings. However, radiation exposure of a fixed amount ensues in order to obtain a high quality image required for a diagnosis of a doctor. On the other hand, a ratio of noise to a signal is increased and a linear artifact and granular noise which would be a cause for an erroneous diagnosis are frequently generated as an exposure dose is reduced in order to realize low-dose exposure. Accordingly, if the artifact and the noise could be reduced in low-dose imaging, a high-quality diagnosis and low-dose exposure can be realized. In order to solve this problem, in a technology of Patent Document 1, an iterative reconstruction method of iteratively correcting a CT image such that measurement projection data becomes equal to calculation projection data is proposed.
On the other hand, in the X-ray CT device, extensive reconstruction is utilized in order to image a local region of a subject required for a diagnosis at a high resolution. However, since the general iterative reconstruction method needs to image the entire subject including a bed, a fixture and the like, extensive reconstruction cannot be applied.
In order to solve this subject, in the document of Non patent, first, a CT image (hereinafter, referred to as a large FOV (Field of View) image) is reconstructed on condition that the entire subject is included. Next, a background image of a background region other than a local region of the subject required for the diagnosis is forward projected, and the obtained background projection data is subtracted from the measurement projection data. Measurement projection data (hereinafter, referred to as local measurement projection data) which is required to iteratively correct a CT image (hereinafter, referred to as a small FOV image) of the local region can be obtained by this processing.